1. Technical Field
The present invention relates to high speed data communication systems having self-test features. More specifically, the present invention provides a communication system with the capability to periodically assure proper performance and receipt of data during variations in the time length of the data window or “eye” of signals in the communication system.
2. Description of the Related Art
In high speed data communication systems, particularly those over a fiberoptic cable, it is desirable both during assembly and installation of the components, as well as periodically during service thereafter, to test these systems and verify proper receipt and performance of data in the system. The term “eye” is a well known communications term used to define the valid data window available to the circuitry that is expected to receive the signal from the cable.
An example of an eye or signal window in ideal form is depicted in a set of eyes 10 occurring repeatedly as a function of time in a signal waveform 12 in FIG. 1. In FIG. 2, an eye 14 is depicted as a function of time of the type representing an example of a signal window actually present in a received waveform 16. Areas 18 and 20 preceding and following the eye 14 represent the effect of noise and other factors which can be present in signals typically present in data transmission network. It can be seen that the waveform time duration of the eye 14 is considerably less than the bit time of the ideal time window or eye 10 in FIG. 1. This is caused by many things that affect the time delay of an actual signal from one end to the other end of the data transmission system.
Jitter is a commonly used term to refer to the time variation between the transmitted bits. Jitter is measured for high speed data transmission in time intervals such as pico seconds (1 pico second=10−12 sec.). If the jitter is too great from one transmitted bit to another transmitted bit, the eye becomes so short as to in effect cease to exist. Data transmission is not then possible either in the equipment under test or over an installed network. It is important to evaluate the response of communication networks and equipment to the effects of jitter. It would thus be desirable to be able to test high speed communication systems, both during assembly and installation and also during subsequent service, to determine the ability of a system and its components to respond to fluctuations or changes in the time length of the data window or “eye” of signals.